Taxonomy & naming
Xenocharax crassus was described by Paul Pellegrin in 1900 from material collected in the Congo basin. For much of the twentieth century it was treated as a junior synonym of the more widely recognised Xenocharax spilurus, a lumping applied by Jacques Daget in 1960 that persisted in most subsequent literature. A later morphometric review, published in Ichthyological Exploration of Freshwaters (vol. 27), re-examined the type material and comparative specimens and rehabilitated X. crassus as a valid, separate species, distinguishing it from X. spilurus on a set of concrete characters detailed below.
The genus Xenocharax sits within the family Distichodontidae — the African characins that also include Distichodus, Nannaethiops and Neolebias — inside the order Characiformes. As with much of this family's taxonomy, the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the currently accepted name and synonymy; this article keeps the genus as Xenocharax per that record and per the rehabilitation paper, and does not attempt to reinterpret it. No common or trade name for X. crassus appears in the available sources.
Morphology
Xenocharax crassus is a thick-set, moderately deep-bodied characin reaching a maximum recorded length of about 7.5 in — a solidly built mid-sized river fish rather than a nano species, though well short of the biggest distichodontids. Fin-ray counts recorded in the rehabilitation literature give 18–22 dorsal soft rays and 13–15 anal soft rays.
The species is separated from its close relative X. spilurus by a combination of features: a large black mark toward the front of the dorsal fin, a small, round black spot at the posterior end of the caudal peduncle, and a proportionally wider mouth, measured at 29–34% of head length versus 24–29% in X. spilurus. A cited voucher specimen (AMNH 241655) measured 5.5 in standard length (about 5.5 in), consistent with a fish that grows somewhat larger still at its recorded maximum.
Habitat
Xenocharax crassus is endemic to the Congo River basin, with its type locality on the Alima River, a tributary within that system. Confirmed range extends across the Republic of the Congo, the Democratic Republic of the Congo and Cameroon. As with other Congo basin distichodontids, it is a warm-water African river fish, though the cached sources do not report specific temperature or pH measurements for this species.
Given the family's general ecology, Congo basin distichodontids are typically found in flowing to slow-moving river channels and associated tributaries, often in tannin-influenced water typical of the wider Congo system, but no site-specific habitat description for X. crassus is available in the sources consulted here.
Feeding
No diet-specific data for Xenocharax crassus was found in the cached sources. Distichodontidae as a family runs from herbivorous grazers to omnivores, and several of its better-studied members — including species of Distichodus — are known plant and algae grazers. Without a direct study of gut contents or feeding ecology for this species, it would be speculative to assign it a specific diet beyond noting the family-wide tendency toward herbivory and omnivory; data is sparse for this particular species.
Mating
No courtship or spawning behaviour has been documented for Xenocharax crassus in the available literature. The species has no history in the aquarium trade and no record of captive husbandry, so there are no hobbyist or aquaculture observations to draw on either. This account does not speculate beyond what the sources support.
Breeding
Breeding biology for Xenocharax crassus is undocumented. Related Congo basin distichodontids such as Distichodus are, as a rule, egg-scatterers that are only rarely bred outside their native rivers, and there is no indication in the cached sources that X. crassus has ever been spawned in captivity. In the absence of direct evidence, the honest position is that this species' reproduction has not been studied or reported, rather than assuming any particular mode.
In the aquarium
Xenocharax crassus has no established presence in the aquarium hobby. There is no trade history, no care guide, and no keeper reports to draw on among the sources consulted. Given its family and recorded size of roughly 7.5 in, any speculative keeping would need to account for a solidly built, mid-sized river fish rather than a small community species, but without direct husbandry data this article makes no specific tank, water-parameter or tankmate recommendations — that information simply does not exist yet for this species.
Conservation
The IUCN Red List assesses Xenocharax crassus as Least Concern. The species is restricted to the Congo basin, occurring in the Republic of the Congo, the Democratic Republic of the Congo and Cameroon, and its LC status reflects a lack of evidence for significant population decline within that endemic range rather than any specific abundance data cited in the sources reviewed here. As with many Congo basin endemics, broader regional pressures on freshwater habitat — though not documented specifically for this species — are worth watching over time even though the fish is not currently considered threatened.